This invention relates to an optical access system and, more particularly, to a technique of executing discovery of optical network units quickly.
The recent propagation of Fiber To The Home (FTTH) which uses an optical fiber has increased the speed of access networks. A representative example of FTTH is a passive optical network (PON) system.
A PON system has, as illustrated in FIG. 2, a plurality of (N) optical network units (ONU) 20, which communicate with user terminals 10, and an optical line terminal (OLT) 40, which communicates with a backbone network 60 via a gateway 50. The plurality of ONUs 20 and the OLT 40 are connected to each other via an optical splitter 80, which is a passive device requiring no power feeding. The PON system can thus implement an inexpensive access system 70.
For instance, IEEE 802.3ah standardizes Ethernet-PONs (EPONs) in which data is transferred between an OLT and at least one ONU in conformity to the Ethernet (registered trademark; hereinafter, Ethernet®). Also, the standardization of 10G-EPONs in which the transmission speed is raised to 10 Gbps is in progress (see IEEE 802.3av).
Discovery processing executed in the optical access system as illustrated in FIG. 2 is processing in which a shared optical communication path is used to detect the ONU 20 that has not been registered, the detected ONU 20 is registered, and the communication distance (Round Trip Time (RTT)) between the ONU 20 and the OLT 40 is measured. The discovery processing allows the optical splitter or optical switch to control communication sessions such that overlapping is avoided. The OLT 40 ultimately discriminates a registered ONU 20 from an unregistered ONU 20 by its logical link ID (LLID). The sequence of a discovery procedure in conventional EPONs and 10G-PONs is illustrated in FIG. 4.
In the discovery processing, the ONUs 20 have not been registered and the OLT 40 first uses a Discovery GATE message SIG20 to check the presence of the ONUs 20 that do not have assigned LLIDs. The Discovery GATE message SIG20 is distinguished from a normal GATE message by setting “1” to a discovery flag in the message. An identifier defined for broadcast is used as the LLID and a multicast address is used as the destination MAC address.
The Discovery GATE message SIG20 sent from the OLT 40 travels through the optical splitter 80 and reaches every ONU 20 that is connected to the optical splitter 80. Receiving the Discovery GATE message SIG20, the unregistered ONUs 20 to which LLIDs have not been assigned each send a REGISTER_REQ message SIG30 in order to request the OLT 40 to execute registration.
The plurality of REGISTER_REQ messages SIG30 have to be prevented from bumping into one another in the section between the optical splitter 80 and the OLT 40, but the collision cannot be avoided completely. To lower the chance of collision, each unregistered ONU 20 sends the REGISTER_REQ message SIG30 at a time point T3, which is reached after a random time period elapses since a transmission start time point T2 written in the Discovery GATE message SIG20.
When the REGISTER_REQ message SIG30 is received within a time period defined as a discovery window, the OLT 40 obtains the MAC address of the ONU 20 from which this REGISTER_REQ message SIG30 has been sent, and manages the association relation between the obtained MAC address of the ONU 20 and an LLID. The OLT 40 also starts processing for assigning the LLID to this ONU 20.
The OLT 40 notifies the assigned LLID to the ONU 20 by sending a REGISTER message SIG40 in which the MAC address of this ONU 20 is set as the destination MAC address and the LLID is written. The ONU 20 that has this destination MAC address receives the REGISTER message SIG40 and obtains the assigned LLID. From then on, the assigned LLID is contained in the preamble of a frame sent from the ONU 20, to thereby enable the OLT 40 to identify the source ONU 20. Also, the LLID contained in the preamble of a frame that is sent from the OLT 40 enables the ONU 20 to determine whether the frame is destined to itself.
Thereafter, in order to measure the round trip time RTT between the OLT 40 and the ONU 20, the OLT 40 sends a GATE message SIG50 in which the ONU 20 is specified by its LLID, a multicast address is set as the destination MAC address, and “0” is set to the discovery flag.
The ONU 20 that has the specified LLID receives the GATE message SIG50 and extracts time information (time stamp) T6 and a transmission start time point (grant start time) T7 from the GATE message SIG50. The time stamp T6 is set to a clock of the ONU 20. When the set clock hits the grant start time T7, the ONU 20 sends a REGISTER_ACK message SIG60 to the OLT 40.
The OLT 40 receives the REGISTER_ACK message SIG60 at a time point T8 by its own clock. From T8 and from T7 contained in the received REGISTER_ACK message SIG60, the OLT 40 calculates the round trip time RTT between the OLT 40 and the ONU 20 (RTT=T8−T7).
Through the sequence described above, the registration (LLID assignment) and measurement of the communication distance RTT are finished for one ONU 20. When there are a plurality of unregistered ONUs, one Discovery GATE message SIG20, a plurality of REGISTER_REQ messages SIG30, a plurality of REGISTER messages SIG40, a plurality of GATE messages SIG50, and a plurality of REGISTER_ACK messages SIG60 are exchanged in a single discovery sequence. A plurality of ONUs are registered by exchanging these messages repeatedly.
How frequently the discovery processing is executed is not regulated by IEEE 802.3ah and IEEE 802.3av, and varies from practice to practice. Commonly, as illustrated in FIG. 3, a given period of time is sectioned into N phases one of which serves as a discovery phase, with remaining N-1 phases serving as data transmission phases, and processing of the given period of time is repeated.